The present invention relates to a fuel system, and more particularly a fuel system for aiding the prevention of the formation of contrails.
Contrails, also known as condensation trails or vapour trails, are line-shaped ice-clouds that appear behind aircraft under certain circumstances. The formation of a contrail depends on a number of factors, including: ambient temperature, humidity and pressure; the efficiency of the aircraft's engines; and the properties of the fuel burned in the engines.
A contrail, once formed, will typically dissipate within a minute or so, unless the ambient air is supersaturated with respect to ice, in which case the contrail may persist. A persistent contrail will grow over time to resemble natural cirrus cloud, both in size and optical properties, and is referred to as “contrail-cirrus”. Line-shaped contrails and contrail-cirrus are collectively referred to as “aviation-induced cloudiness” (AIC).
It has been found that contrail formation may be affected by the properties of the fuel that is burned in the engine of an aircraft. Kerosene and other hydrocarbon fuels typically contain a wide variety of types of molecule, characterised by their sizes (number of carbon atoms) and shapes. Some common molecule shapes include: chain-shaped molecules (paraffins), chains with branches (iso-paraffins), and chains wrapped into rings (cyclo-paraffins). Also common within many hydrocarbon fuels is the family of molecules known as “aromatics”, which are also ring-shaped but possess different properties from the cyclo-paraffins.
There is anticipation in the aviation industry of a trend towards the use of fuels with a lower aromatic content, as a low aromatic content may provide many benefits, such as a higher specific energy, lower soot emissions, and lower CO2 emissions relative to fossil kerosene.
Although lower aromatic fuels may have a number of advantages over fossil kerosene, their incorporation into jet fuel nonetheless presents a problem regarding the susceptibility of an engine to the formation of contrails, since the lower aromatic content of the fuel means that the ratio of water-vapour-to-heat added by the engine to the exhaust plume is increased. This enables the formation of contrails over a wider range of atmospheric conditions, resulting in increased prevalence of contrails.
Depending on the metric employed, the climate-warming impact of aviation-induced cloudiness may be of a similar magnitude to that of the CO2 emitted by aircraft, and may therefore represent a significant element of aviation's total climate impact. The suppression of contrail formation, and particularly the suppression of persistent contrails, therefore represents a compelling opportunity for a significant reduction in the overall climate warming impact of aviation.
“On conditions for contrail formation from aircraft exhausts”, Meteorol Z, N F 5, Schumann (1996) discusses the relevance of fuel properties to the formation of contrails, and in particular that fuels such as liquefied natural gas or hydrogen are more susceptible to contrail formation than kerosene.
US2008/0072577A (Rolls-Royce) describes the suppression of contrail formation through the removal of water vapour from the exhaust, making use of a heat-exchanger and condenser arrangement which is integrated with intercooling and recuperation. US2010/0132330A (Rolls-Royce) proposes the attempted suppression of contrail formation through the use of directed electromagnetic energy which is applied to the engine's exhaust plume. Each of these methods may however result in a significant weight penalty for the engine. Furthermore, the latter example would also need to draw power thereby reducing fuel efficiency and, particularly for military applications, the emission of electromagnetic radiation may have the undesirable effect of increasing aircraft detectability.
Other solutions include eliminating some or all contrail formation and/or persistence through routing aircraft around/above/below regions of air susceptible to contrail formation and/or persistence, yet such a solution results in a wide variety of further issues to be solved. For example, rerouting of aircraft to avoid regions prone to contrail persistence may cause associated air traffic control complications as well as the increased fuel burn involved in climbing or otherwise increasing the distance travelled by an aircraft and/or flying at a non-optimal cruise altitude.
It is therefore an object of the present invention to reduce and/or eliminate the formation of contrails by aircraft engines in a manner that mitigates or avoids some or all of the problems that result from the prior art methods discussed above. It may be considered an additional or alternative aim of the present invention to reduce the production by aircraft engines of pollutants such as soot which may adversely impact local air quality around airports. It may be considered an additional or alternative aim to provide a system for aircraft engine contrail suppression which can operate more efficiently or effectively than the prior art.